1. Field of the Invention
The present invention relates to lasers and particularly to a diode-pumped, fiber laser doped with erbium activator ions for producing an output CW laser emission at a wavelength of substantially 2.7 microns.
2. Description of the Prior Art
In low power applications, such as in telecommunications and in medical and sensing applications, the use of fiber lasers is becoming more and more important.
In a typical fiber laser a rare earth, such as erbium, neodymium, terbium or praseodymium, is doped into the core of an optical fiber to provide an active gain medium for the fiber laser. Typically, the optical fiber is comprised of silica. The input end of the fiber laser is pumped with optical radiation to produce lasing action in the fiber laser at a wavelength essentially determined by the dopant and the mirror reflectivities. The doped optical fiber is included in the laser resonant cavity of the fiber laser.
A major disadvantage of using a silica fiber as the host optical fiber for the dopant rare earth is that a silica fiber is not suitable for transmitting wavelengths longer than 2 microns. The reason for this is that there is too much attenuation of light in the silica fiber at wavelengths above 2 microns.
Intense research and development have been conducted in the area of fluorozirconate (ZrBaLaNa or ZBLAN) glasses to produce ultra-low-loss fibers for optical communications. Minimum transmission losses in ZBLAN fibers occur over the wavelength range between 2 and 3 microns. It is, therefore, highly desirable to develop ZBLAN rare earth fiber lasers in this wavelength range.
A 2.7 micron, erbium-doped multimode fiber laser pumped by a tunable argon laser has been reported by M. C. Brierley et al. in Elect. Letters 24, page 15 (1988). More recently, a 2.71 micron, erbium-doped fluorozirconate single-mode fiber laser pumped by a tunable argon laser at 476.5 and 501.7 nm was reported by J. Y. Allain et al. in Elect. Letters 25(1), page 28 (1989). It was concluded by Brierley et al. and Allain et al., as well as by R. S. Quimby et al. (Applied Optics 28(1), Page 14 (1989)), that excited state absorption (ESA) of the pump beam by the .sup.4 I.sub.13/2 terminal level was the mechanism which allowed population inversion between the .sup.4 I.sub.11/2 and .sup.4 I.sub.13/2. The laser transition is from the .sup.4 I.sub.11/2 level to the .sup.4 I.sub.13/2 level in trivalent erbium (Er.sup.3+), in which the lower level lifetime (9.4 milliseconds) is longer than the upper level lifetime (7.5 milliseconds)
The argon ion laser is a large and inefficient pump source for practical applications. It would be desirable to use laser diode pumping for a compact and efficient laser system. However, the question arises as to whether some processes such as ESA from the .sup.4 I.sub.13/2 level will be necessary to achieve lasing when pumping around 800 nanometers (nm) using a laser diode pump source. Quimby et al. measured the transition rates involved and concluded that CW lasing would be possible based on favorable branching ratios from the .sup.4 I.sub.11/2 level or manifold, when the .sup.4 I.sub.11/2 manifold is populated by pumping either into the .sup.4 F.sub.9/2 level or .sup.4 I.sub.9/2 level. Quimby et al. also determined that the bright green fluorescence observed when pumping at 800 nm is actually an inefficient upconversion process which should have little or no effect on laser performance in a fiber because of the relatively low erbium concentration used. F. Auzel et al. (in Elect. Letters 24, page 909 (1988)) measured the laser cross-section and quantum yield of the 2.7 micron transition. However, F. Auzel et al. concluded that CW lasing in a fluorozirconate glass host is not likely to occur without some additional means of reducing the lifetime of the .sup.4 I.sub.13/2 terminal laser level.